Electric discharge device



April 6, 1937 J w RYDE ET AL 2,076,278

ELECTRIC DISCHARGE DEVICE Filed Sept. 7, 19:55

FIG. ll

'//VV/V7'0k5 R 3 JOHN WALTER RYDE JULIUS GATES ATTORNEY Patented Apr. 6,1937 I UNITED STATES ELECTRIC DISCHARGE DEVICE John Walter Ryde andJulius Oates, London, England,'assignors to General Electric Company, acorporation of New York Application September 7, 1935, Serial No. 89,640

In Great Britain September 7, 1934 Claims.

This invention relates to a circuit arrangement for operating electricdischarge devices of the high-pressure metal-vapour type. Since itdepends on certain operating characteristics of de- 5 vices of thistype, and not on their content, luminous emission, or other features,the type is defined for the purpose of the invention by the followingstatement:----

Let VG be the voltage across the device necessary to start a dischargein it when the device is cold (that is to say, first brought intooperation) V21, the voltage necessary to start a discharge when thedevice is hot (that is to say, when it has been operating for so longthat it has reached a state of thermal equilibrium, is thenextinguished,

and the starting voltageapplied to it after an interval just sufficientto allow the ions to disappear); V1, the voltage across the deviceimmediately after the discharge has been started .0 from cold; V2, thevoltage across-it when the device has operated for so long that it is inthermal equilibrium. Then the device is of the highpressure metal-vapourtype if, and only if, both V0 and V2 are much greater than V1; and Vh is25 much greater than Vc.

The voltages mentioned in this definition are not entirely precise. For,if the device is operated (as is usual) on A. C., and the values of theVs are R. M. S. values, they will depend on the wave- 30 form of thesupply and. may be different from the D. C. values; for V0 and Vs dependon the peak rather than on the mean voltage of the supply. Again, evenon a D. C. supply, Vc and Vn are not perfectly definite; there is apt tobe a .range of voltage within which starting is uncertain. Lastly V1 andV2 depend on the circuit in which the device is connected, as well as onthe device itself.

In what follows we shall have to take account of these complications.But they donot introduce any practical difficulties in deciding whethera device satisfies the definition. For this purpose it is sufiicient totake R. M. S. values for an approximately sinusoidal supply, and toestimate V1 and V2 when the deviceis in any circuit by which it isadapted to be normally operated. Thus, on this understanding, a typicaldevice to which the invention relates may have the following values:-

Viz-20 volts.

Vz=150 volts.

V0: volts.

Vh=more than 300 volts.

The difference between'Vn and Vc is apt to lead to serious inconvenienceif the supply is liable to interruptions or sudden decreases of voltage,even if they are momentary. For, if the normal supply voltage liesbetween Vc and Vh, the device, extinguished by a lowering of the supplyvoltage, is not restarted immediately the 'or from a constant currenttransformer.

(Cl. 176-124) u voltage is restored; it will not start again until thedevice has cooled down so that the starting voltage again approaches Vc.If the device is a lamp (as it usually is), a momentary interruption ofthe supply may cause a considerable period of darkness; and, even if itsprimary purpose is not to supply light, other disadvantages may arisethat are of particular importance in connection with the constantcurrent circuits that will be considered more fully later. These dis-'advantages mayarise equally if the device is extinguished by its own(merely temporary) failure, rather than by failure of thesupply.

It has been proposed to overcome the difiiculty arising from failure ofthe supply by connecting a pair of similar high-pressure metal-vapourdischarge devices in parallel. Only one (say A) will strike when thesupply voltage is first applied; for they can never be exactly similar.The other (say B) will remain cool while A operates. If A --isextinguished, B will strike as soon as the supply period, the lightcomes from B and not from A.

Accordingly matters have to be arranged so that it is immaterial whetherA or B is running normally. This involves either a lighting fixture inwhich two lamps can be placed in equivalent positions or two separatefixtures similarly related to the street; both these'alternatives areoften inconvenient.

The same objection applies toanother solution of the problem, suggestedby a common practice when incandescent lamps are run either in seriesThis is to make the starting voltage of B not greater than that of A,and to place in series with it an insulator which breaks down andconducts when a sufficient' voltage is applied to it. A will then alwaysstart from cold in preference to B; but if A is hot, or for some otherreason will not start at its normal Vc, the voltage across B andits-insulator in series may be sufiicient both to break down theinsulator and to .start B. But here again B continues in operation afterA has returned to normality. A further objection is that it is difiicultto make this device work with certainty in connection with dischargetubes.

The object of the invention is to overcome this objection and to providemeans whereby, when A is extinguished and will not restart at its normalvoltage (owing to temporary failure of itself or the supply), analternative path is provided for the current normally flowing through A(or for current of approximately the same magnitude), and whereby also,when A is restored to normality, the current will leave'the alternativepath and A will operate once more. If A is a lamp, it is obviouslydesirable that the alternative path should produce light, and in somecircumstances it is desirable further that this light should be of 10the same character as that from A, and should therefore come fromanother lamp of the same type in the alternative path. But it is notessential to the invention that these further objects should always beattained.

15 We have found that the object of the invention can be attained byconstructing the alternative path of a continuous impedance (hereaftertermed R) in series with a discharge tube (hereafter called B), thisalternative path being con- 20 nected directly across the terminals ofthe normally operating high-pressure metal-vapour discharge devices(hereafter termed A), so that both A and the alternative path are inseries with the stabilizing impedance (or its equivalent) 25 which isalways required in series with high-presis not, restored to thenon-conducting state when.

the potential that produced conduction is re- 40 moved.

Of course there is nothing new in proposing generally to shunt onedischarge device (not necessarily a high-pressure metal-vapour device)by another discharge device in series with a continuous impedance. Inorder that such arrangement should lie within the invention, theelements A, B, R must fulfill certain conditions. The fact on which theinvention rests is that, owing to the special properties ofhigh-pressure metal-vapour discharge devices, as defined, it is possibleto secure fulfilment of these conditions so as to secure the object ofthe invention.

We will first state more formally the conditions that have to befulfilled in operation, in order that 55 the object of the inventionshall be attained.

They are:

(1) So long as the supply voltage is normal, a discharge must passeither through A or through the alternative path and therefore throughB.

60 (2) When the normal supply voltage is first applied the dischargestarts in A within some brief period, but the discharge may start firstin B.

(3) If, when the normal voltage is first-applied,

the discharge starts first in B, then the starting 65 of A willextinguish B.

(4) B cannot start at any time after the starting of A, so long as Aremains in operation.

(5) If B is in full operation because'A has been extinguished, either byits own failure or that of 70 the supply, A will start again andextinguish B when A is restored to normality.

Sometimes, but not always, another condition is necessary, namely (6)that the impedance of B and R in series, when passing current, is not 75very diiferent from that of Awhen passing cur- Actually it is veryseldom desirable to operate high-pressure metal-vapour devices on D. C.,because energy has then to be wasted in the stabilizing impedance, whichhas to be a resistance. But the supposition enables the complicationsabove mentioned, arising from the effect of varying wave-form, to beignored.

Some further definitions must be introduced.

Voltages characteristic of A will be distinguished by the sufiix M (formain) they denote voltages across the terminals of A, and do not includethe voltage across the stabilizing impedance (or its equivalent). Thecorresponding voltages characteristic of the alternative path will bedenoted by the sufiix S (for subsidiary) they are voltages across thecombination consisting of R and B in series. Since no current is flowingwhen V is estimated, (Vc)S is the cold starting voltage of B alone.(V2)s is the voltage across the combination when it has reached thermalequilibrium. (V1)s is the voltage across the combination immediatelyafter B starts (or, more accurately, the maximum voltage attained withinsome inappreciable interval after starting). (Vm)s is the minimumvoltage across the combination which enables a current to pass throughit at all when A is passing a discharge. In virtue of the fact that B isa discharge device, there will always be such a minimum voltage in anydeterminate conditions. E is the normal supply voltage. I

In order that the operating conditions (1)-(5) abovementioned should besatisfied, it is sufficient (subject to comments made below) that all ofthe following statements should be true:-

(1) (vc)M and (Vc)S are both less than E.

(2) Either (Vc)s is greater than (vnm, or (V1) s (V0) M.

(3) (Vm)s is greater than (V1)M.

(4) (Vc)s is greater than (Vzhvr.

(') (V2)s is greater than (VOM.

The condition that all these statements should be true will be calledcondition P, when the supply is D. C.

Examination will show that the truth of each of these statements meansapproximately that the operating condition similarly numbered issatisfied: No proof of this statement will be necessary for thoseskilled in the art; but it is to be observed that condition (5), as wellas condition (3) requires that (3) should be true.

However, the following comments are necessary. First, (1') may notensure that (1) is satisfied if it can happen that both A and B are hotat the same moment and therefore have starting voltages higher than therespective Vc.

to be troublesome; but it may sometimes be necessary to impose a rathermore stringent condition than P. However, condition P must still befulfilled. 1

Second, we have to take into account the in consistency aforementionedin the values of the characteristic voltages and especially in the Vcs.

Vt may vary with temperature and with the age of the device. Thisinconsistency has to be allowed for by means of a factor of safety; thatis to say, it is necessary in practice (for example in (1')) that (V014and (Vc)S should not be merely less than E, but less by someconsiderable margin. The margins that are suitable will appear from theexamples given below. Condition P is to be understood as including asuitable factor of safety.

So far only D. C. operation has been considered. When the supply is A.C., .the statements (1') to (5') have no definite meaning; for they maybe true (e. g.) if peak values are used and untrue (e: g.) if R. M. S.values are used. In order to produce definite and accurate statements ofthe same form, it would be necessary to consider the variation of each Vthrough a cycle and to take account of the fact that the waveform of thevoltage applied to B .(or A) may depend on whether current is or is notpassing in A (or B). Accordingly it is desirable to give the statementsa different form. But, before the alternative statements are given, itshould be pointed out that, if the supply is sinusoidal, and if thefactor of safety, necessary on the score of consistency, is allowed,then condition P in the form already given will often secure conditions(1) to (5), when R. M. S. values are used throughout for the Vs and forE. Thus in these conditions (Vc)M may be identified with the R. M. S.value of the least sinusoidal voltage which will start a dischargeconsistently in A; (V1)s with the R. M. S. value of the voltage 40across the alternative path just after the discharge in B has started,the supply voltage being sinusoidal; and so on. 7

The alternative statements describe certain tests to be applied to thecircuit. These tests will now be'des'cribed with reference to Figure 3of the accompanying drawing. As shown in this figure, the components areconnected in a main and alternative path across the actualA. C. supplyof voltage. E to be used, the stabilizing impedance Q being in serieswith both paths. The main path contains a make-and-break switch m, thealternative path a make-and-break switch S; a make-and-break switch 11.is in series with both paths. The following tests are made alwaysstarting in each case with all switches open, and the devices A and Bcold; if the result indicated in each case is obtained, condition P issatisfied.

Test 1a. Close m; close 11. Result: A starts. 1b. Close S; close n.Result: B starts.

Examination will show that the tests la, 1b secure that condition (1) issatisfied; 2a, 2b that condition (2) is satisfied, and so on. It is tobe observed that, if condition P in its earlier form for a D. C. supplyis satisfied, the tests would succeed if a D. 0. supply were used forthem. Condition P, defined by the tests, is therefore sufficient for anysupply, whether A. C. or D. C.

According to the invention a circuit arrangement for operating one ormore high-pressure metal-vapour electric discharge devices (A) com--prises an alternative path connected across the terminals of at leastone of the said devices A, the said path consisting of a dischargedevice (B) in series with a continuous impedance (R), the voltagescharacteristic of A, B, R being so related to each other'andcto thenormal supply voltage that condition P is satisfied in actual operation.

Condition (6) must now be considered. Here three known circuitarrangements for operating the devices must be distinguished. (a) Anarrangement in which the devices are connected in parallel across mainsof low impedance, (b) an arrangement in which they are fed by a commonconstant current transformer, being connected across the secondaries ofseparate transformers whose primaries are all in series ,with thesecondary of the constant current transformer, (c) an arrangement inwhich they are connected across the secondaries of transformers (whichare usually leaky transformers) whose primaries are all in series with aconstant potential supply.

In arrangement (b) it is always necessary that 1 condition (6) should befulfilled; the exact degree to which the impedances of the main andalternative paths must be similar depends on the nature of the constantcurrent transformer. In (0) it is usually necessary; but the degree ofsimilarity in the impedances is apt to be less.

In (a) it is not necessary at all, unless it is required that thealternative path should consume, for the purpose of emitting light,watts not very different from those consumed in the main circuit. Thedegree of similarity of the impedances will then depend on the amount oflight required and on the means in the alternative path for emittinglight.

If condition (6) has to be fulfilled, it will, of

course, impose additional restrictions on the choice of the components Band R. The nature of these restrictions will be clear to those skilledin the art in the light of the examples given below. But some generalobservations will be offered here.

If one of theobjects of the invention is to provide a source of lightalternative to A, when A is out of operation, and emitting light of thesame character as A, then B must be another highpressure metal-vapourdischarge device. It is then just possible that the impedance R might be.omitted, that is to say, its value might ,be zero. For it is sometimespossible to find pairs of such discharge devices which will fulfillcondition P when sufiix S denotes the voltage across B alone. But evenin this case it is preferable that impedance R should befinite; for thesatis fying of condition P is then much easier and more certain, and theimpedance, if it is approximately a pure reactance, need waste noappreciable amount of energy.

not provide light similar to that from A, B need -not be a high-pressuremetal-vapour discharge device. Even if the alternative path has toprovide light, it is usually preferable to make R an incandescent lampand to obtainthe light from.

this lamp rather than from B.- "It is then desirable that the voltageacross B and'the energy consumed in it-should be kept as small aspossible. B servesmerely to control the starting and stopping of thecurrentin the alternative path.

Certain embodiments of the invention will now be described byway ofexample, each corresponding to a different requirement.

In all cases the supply circuit is 50 cycle A. C.

A is a hi gh-pressuremercury vapour lamp of the. known type adapted toconsume 400 watts on a main supplyof about .230 R. M. s. volts. In thefirst threeexamples the supply voltage is drawn from mains of lowimpedance (arrangement (a) A number of similar lamps A, each with itsalternative path, are connected across the mains.

This-arrangement; is shown in Figure l of the accompanying drawing.-Here II and 2 are-the mains of voltage E, 3 the stabilizing impedance,

usually a choke, A, B,' 'R the components so placed across the mains forcorrecting the power denoted in the statementv of. the invention. R isshown as a choke in the figure; but, as will appear below, it maybe'either a choke, a condenser, or a. resistor. The condensers usuallyfactor are not shown; of course theyaredesir able, but they do notaffect the invention.

In the first example nothing is required of the condition P; but somelimitation-will be imposed power factor of about 0.6. I

by condition (6).

A may be characterized by the voltages already given as -an example,namely (Vt)M=1s0; vh M 30 vo =2o;(vi M=150;

' 'E= 2 3 0 "vo1ts The "stabilizing. choke 3 has an imp of 50 ohms. Thecurrent passing through A in the state characterized by V2 is about2.9A. The A with its stabilizing choke is Rv is a tungsten gas-filleddesigned for 200 watts at 210 volts.

.B isa dischargedevice similar in constructio toztheinner tubeof A; {butit is rather smaller and has noouter. envelopei .-V1 and vz' for -this"device are thenfmuchwrnnre nearly equal than they;are in A-; ,for..tvapour pres' 'sure' of the mercury does n'ot-Erise much in ope 'tion.(The same result may-be attained byf est-rioting the aforesaid R, thecharacteristic supply for, mercury in B, or means) Such .a device cartthat, when it is .used in -com onstructed so ation with v the tages'ofthe alternative path are (V )s 180; (voszzoo; (v2

The state characterized by (V1)S=2d.0 is reached as soonas the tungstenlamp has heated up;

the further rise to (Vz) j joccupies only a few sec onds. In thisequilibriumstate, 0.93 amp. passes only 14.5 are acrossB.

through the alternative 5 incandenscent lamp ther :known I a of the 218volts- Condition P is thus iuifin t; Since (yes is not greater than(Vtl-M, either-A or V may strike so'that-A will strike, even ir st kesfirst; and, since (Vm)s is greater than (V1)M, *Awill extinguish B whenit strikes. Y j

In the second and third examples, it is required that the light from thealternative path. should be of the same quality as that from A. -.B musttherefore be a high-pressure mercurvdischarge lamp, and R is preferablyreactive. :I "the seeond example it is a choke, in the third it is acon; denser; a condenser turns out to be imor'e convenient; for in orderto meet-the new requirements with a choke for R, it is desirable to.give the lamp A slightly different characteristics;

moreover the factor of safety is greater with the condenser.

In the second example The stabilizing-choke 3 has animpe dance of 19ohms.

R is an inductance of ohms impedance;

B isa 100 watt high-pressure mercury-vapour.

lamp. on starting from cold (with Aout) its v1 is 19.volts; the currentthrough it is A; 24b watts are consumed init and the voltage across thechoke 3 is 67. When it is hot (withA out), its V2 is 117 volts; thecurrent about 1A; the watts 100; 'the volts across choke 3 are ,51.Withsuch a lamp we have current 124A; watts 21; voltage across choke 3about 62. When it is hot, its V2 is volts; current 1AA; watts 159;voltage across choke 3 about '15. With such a lamp we have It will beobserved that the substitution of a condenser for a choke has theadvantage-of raising (Vi)s; but' it' will probablydecrease the powerfactor when the alternative path istaking the current. It is here,assumed that thepower factor is corrected when the main path is inoperation. 1

In the fourth and fifth examples, the supply circuit is different;the'lamps are fed from a constant current transformer (arrangement (2))the advantages of this form of supply in a street lighting system arewell known. The arrangement appropriate to this case is shown in Figure2 ofv the accompanying drawing. Here i is a transformer designed toprovide a constant current Ii=(say)6.6A. It supplies a plurality ofseries connected primaries of ordinary transformers 2, whose secondariesfeed the lamps A; the

current through these secondaries tends to a of the correspondingprimary may become so high that the transformer I can no longer maintainthe constant current L; the remaining lamps may be extinguished orunderrun. Even if the transformer i can eventually maintain the currentI1, it will need time to adjust itself; and during this time-the lamps Amay be extinguished, so that they will not start again when theadjustment is complete.

In these circumstances the use of the invention is particularlydesirable. The alternative paths B, R are provided across each lamp A.In order to obtain full advantage of it, Bmust start so quickly after Ais extinguished that I1 has no time to fall. There is usually nodifliculty in fulfilling this condition. It is now necessary that theimpedances of the main and alternative paths should be closely similar.

In the fourth example, the light provided by the alternative path comesfrom an incandescent lamp, as in the first example. Then B is requiredmerely to start and stop the current in the alternative path.

J The constant current transformer l is designed to yield a constantcurrent I1=6.6A in the primary circuits of the transformers 2 and about3.1A in their secondary circuits. If the secondary circuits are open,the secondary volts rise to 4'70; consequently E is not much less than470. l The lamps A are characterized by R is made up of an incandescentlamp, designed to consume 198 watts with 65 volts across it, in serieswith a' choke having an impedance of 54 ohms. This choke enables a largevoltage to be developed across S without a large wattage being consumedin it.

B is a discharge device of the same general 40 kind as in the firstexample, so that its V1 and V2 are nearly equal. Then it can be chosenso that 45 About 44 watts will be consumed in B and about 198 in thelamp.

In the fifth example, the light from the alternative path is provided bya high-pressure mercury lamp B; R is therefore preferably purely 50reactive. With the same transformers and the same characteristics of Aas in the fourth example, R may be a choke of about '70 ohms impedanceand B a 250 watt high-pressure mercury lamp. In this lamp on startingV1=l9 volts; the

55 current is 3.11A;' the watts 50. When hot, V2=10D, the current is3.09A; the watts 280. Then the following values for S can be obtained(Vc) s=180; (V1) 5:212; (V2) 5:247; (Vm) 5 30-40 Certain irregularities,not fully explained, have been observed in the operation of the circuitin this fifth example. Thus, after B starts from cold, the currentthrough it may fall and then rise again. They probably arise from thepresence of the choke in series with B; for they do not arise in A. But,except in abnormal circumstances, they do not aifect the attainmentofthe object of the invention.

In any circuit arrangement according to the invention, the alternativepath is a mere stop-gap, and is not intended to be operated normally.Accordingly if it provides light, there is no need to place the sourcein a position optically equivalent 75 to that of A. But in astreetlighting system it will generally be convenient to place it on thesame post as A.

If arrangement (c) is used for supplying the devices, the combination ofB and R required will in general be more similar to that suitable forarrangement.(b) than to that suitable for arrangement (a) In any of thearrangements (a), (b) or (c) it is not necessary that if the alternativepath provides light, this light should be comparable in amount with thatproduced in the main path. Thus the alternative path may provide a merepilot light to indicate to a supervisor that A is extinguished. In thiscase the watts consumed in the alternative pathmay be small.. Thus R maybe purely reactive and B be adischarge lamp consuming few watts, whichserves as the pilot light. Alternatively R may be mainly reactive, butincludes a small wattage incandescent lamp, which is used as the pilot;then B need not be used to give light at all.

We claim:

1. In combination, a main electric gaseous discharge device of a typewhose starting potential at normal operating temperatureis far in excessof its starting potential at room temperature, and whose dischargemaintaining potential lies within a fixed range, an impedance betweensaid device and a source of constant potential, and an im-' pedance inparallel with said device consisting at least in part of a secondgaseousdischarge device whose starting potential is in excess of saidrange but below the hot starting potential of said main device, thevoltage drop attained across the latter impedance when said seconddischarge device reaches a given temperature in operation being inexcess of the room temperature starting potential of said main device,the minimum potential required to maintain current flow through saidlatter impedance whemsaid see- 0nd device is at said given temperaturebeing in excess of the maintaining potential of said main device when itrestarts, whereby said second device serves as a standby for the maindevice, functioning for an interval whenever potential is appliedthereto after the discharge in the main device has been extinguishedafter it is appreciably above room'temperature and automaticallytransferring the discharge back to said main device when the latterapproaches room temperature provided said main device is in operablecondition.

2. In combination, a main electric gaseous discharge device of a typewhose starting potential at normal operating temperature is far inexcess of its starting potential at room temperature, and whosedischarge maintaining potential .lies within a fixed range, an impedancebetweensaid device and a source of constant potential, and

- required to maintain current flow through. said latter impedancewhen=said second device is at said given temperature being in excess ofthe maintaining potential of said main device'when it restarts, wherebysaid second device serves as a standby for the main device, functioningfor an interval whenever potential is applied thereto after thedischarge in the main device has been extinguished after it isappreciably above room temperature and automatically transferring thedischarge back to said main device when the latter "approaches roomtemperature provided said main device is in operable condition.

3. In combination, a main electric gaseous discharge device of a typewhose starting potential at normal operating temperature is far inexcess of its starting potential at room temperature, and whosedischarge maintaining potential lies within a fixed range, an impedancebetween said device and a source of constant potential, and an impedancein parallel with said device, said impedance consisting at least in partof a second gaseous discharge device of the same type as said maindischarge device but whose starting potential is in excess of said rangebut below the hot startingpotential of said main device, the voltagedrop attained across the latter impedance when said second dischargedevice reaches a given temperature in operation being in excess of theroom temperature starting potential of said main device, the minimumpotential required to maintain current flow through said latterimpedance when said second device is at said given temperature being inexcess of .the maintaining potential of said main device when itrestarts, whereby said second device serves as a standby for the maindevice, functioning for an interval whenever potential is appliedthereto after the discharge in the main device has been extinguishedafter it is appreciably above room temperature and auto-' maticallytransferring the discharge back to said main device when the latterapproaches room temperature provided said main device is in operablecondition.

4. In combination, a main electric gaseous discharge device of a typewhose starting potential at normal operating temperature is far inexcess of its starting potential at room temperature,

'- and whose discharge maintaining potential lies within a fixed range,an impedance between said device and a source of constant potential, andan impedance in parallel with said device, said impedance consisting ofa condenser and a second gaseous'discharge device of the same type assaid main discharge device in series therewith whose starting potentialis in excess of said range but below the hot starting potential of saidmain device, the voltage drop attained across the latter impedance whensaid second discharge device reaches a given temperature in operationbeing in excess of the room temperature starting potential of said maindevice, the minimum potential required to maintain current flow throughsaid latter impedance when said second device is at said giventemperature being in excess of the maintaining potential of said maindevice when it restarts, whereby said second device serves as a standbyfor the main device, functioning for an interval whenever potential isapplied thereto after the discharge in the main device has beenextinguished after it is appreciably above room temperature andautomatically transferring the discharge back to said main device whenthe latter approaches room temperature provided said main device is inoperable condition.

5. In combination, a main electric gaseous discharge device of a typewhose starting potential at normal operating temperature is far inexcess of its starting potential at room temperature, and whosedischarge maintaining potential lies within a fixed range, an impedancebetweensaid device and a source of constant potential, and an impedanceinparallel with said device, said impedance comprising an incandescentlamp in series with a second gaseous discharge device Whose startingpotential is in excess of said range but below the hot startingpotential of said main device, the voltage drop attained across thelatter impedance when said second discharge device reaches a giventemperature in operation being in excess of the room temperaturestarting potential of said main device, the minimum potential requiredto maintain current flow through said latter impedance when said seconddevice is at said given temperature being in excess of the maintainingpotential of said main device when it restarts, whereby said seconddevice and the incandescent lamp serve as a standby for the main device,functioning for an interval whenever potential is applied thereto afterthe discharge in the main device has been extinguished after it isappreciably above room temperature and automatically transferring thedischarge back to said main device when the latter appreaches roomtemperature provided said main device is in operable condition.

JOHN WALTER RYDE. JULIUS CATES.

